Another way, which I don't think was mentioned above, which is what I usually do, is to go to a PinPics# that you want, and at the bottom of the page, click on "What could I trade to get this pin?"
It will bring up the list of pins that you have for trade, that people who have the pin you want have for trade. In other words, it brings up the pins that you could offer as a trade to people.
This is very important: check the little boxes for the pins that you think would be a FAIR trade request, taking into consideration the edition size, Trades/Wants ratio, source, and overall value of each pin. As a general rule, do not offer an open edition pin for an LE pin, etc. There are exceptions to this rule, but this will get you started.
If you don't know what I mean by Trades/Wants ratio, here is a little bit about that. To simplify it a bit, if your pin has a low number of people trading it, and a high number of people wanting it, that is a good T/W ratio. If it has a high number of people trading it, and a low number of people wanting it, that is not such a good T/W ratio. Theoretically, you should offer pins of similar edition size, and similar T/W ratio. But let's say their pin has a better edition size, but your pin has a better T/W ratio, they still may consider the trade. Just use common sense, and think to yourself, "Would I think this is a fair trade, if I received it in my email inbox?" It isn't always perfect, but do the best you can. If you have questions about what would be a fair trade, feel free to ask the board.
Not to make things complicated, but another way to do this same thing is to go to the PinPics# of a pin you have for trade, and at the bottom of the page, click on "What could I get for trading this pin". It will give you a list, just like above, but it will be the opposite. You will check the little boxes for the pins you would be willing to receive for your trader, making sure to still only check the ones that you think are fair offers.
PinPics will then send emails to those people who have the pin you want, who want the pin you have. It will let them know that you are offering the pin you have for trade, in exchange for the pin they have for trade. They may say yes, no, or give you a counter offer.
Once you get your first "yes" response and you are ready to make a trade, read my instructions for trading on this thread:
http://disneypinforum.com/showthread.php?9012-Pinpics-trading-
ETA: I see now where JKrolak did talk about this method earlier. Sorry.